I use Advertising Manager on my site for AdSense and haven't had any problems with it. It has a useful import feature where you just paste your AdSense code and it will automatically configure the appropriate ad settings.

To insert an ad into the header using Advertising manager, you could simply edit your theme and use the following code:

<?php advman_ad('YOUR-ADVERTISMENT-NAME'); ?>

Advertising Manager also offers a drop-down selector for inserting ads into posts and a sidebar widget.